1. Field of the Invention
The present invention is related to the field of gas sensing devices and more particularly to that portion of the above-noted field concerned with electrical resistive devices whose resistance varies as a gas to which it is exposed varies. More particularly still, the present invention is related to that portion of the above-noted field concerned with ceramic gas sensing devices whose internal resistance varies in response to variations in the partial pressure of oxygen in the gaseous environment surrounding the device.
2. Description of the Prior Art
The temperature of the exhaust gases leaving the combustion chambers or reciprocating internal combustion engines is proportional to the amount of combustion taking place within the engine and this relationship has been used in aircraft for indicating the air-fuel ratio of the combustible mixture being supplied to the engine. Subsequent investigations showed that the thermal conductivities of various exhaust gas components could be used to indicate the proportion of such components in the exhaust gases. These investigations produced systems of the resistance bridge type that compared the thermal conductivity of the exhaust gases with known gas mixtures to indicate either air-fuel ratio or the combustion efficiency of the engine.
Recent interest in improving the environment by diminishing the quantity of undesirable components in the exhaust gases of automotive engines has accentuated investigations into systems for monitoring continuously the air-fuel ratio of combustible mixtures. These investigations have led to numerous refinements of the thermal conductivity system. For example, it was found that thermal conductivity varies almost linearly with the carbon dioxide content of the exhaust gases and carbon dioxide content in turn is proportional to the air-fuel ratio. Subsequently it was found that the thermal conductivity of the exhaust gases is a function of both the carbon dioxide content and the hydrogen content. Other approaches involved combining thermal conductivity devices with exhaust gas temperature devices.
Systems have been suggested for determining the air-fuel ratio of the mixture supplied to a combustion mechanism by detecting directly the oxidation-reduction characteristics of the exhaust gases. The system comprises a sensing member that is located in contact with either the air-fuel mixture supplied to the combustion mechanism or the exhaust gases leaving the mechanism. Two electrodes spaced apart from each other by at least a portion of the sensing member are attached to the member and to an electrical or electronic device for sensing the electrical resistance across the electrodes. The electrical resistance is proportional to the equilibrium oxygen pressure of the gaseous mixture in contact therewith and resistance measurements can be converted directly into the air-fuel ratio of the mixture supplied to the combustion mechanism.
Equilibrium oxygen pressure is the partial pressure of the oxygen in a gaseous mixture when the mixture is brought to complete chemical equilibrium. The system would thus measure equilibrium oxygen pressure of a gaseous mixture even though the gaseous mixture is not at chemical equilibrium, i.e., even though the actual pressure of the oxygen exceeds the partial pressure that would be present at equilibrium.
Sensing members for the air-fuel ratio control systems are preferably located in the exhaust gases leaving the combustion mechanism because the exhaust gases approximate more closely the desired operating temperatures of the members and do not contain any unvaporized fuel. The system is useful particularly in measuring and controlling the air-fuel ratio of the combustible mixture being supplied to an internal combustion engine.
The sensing member preferably is a relatively thin plate made from sintered particles of the desired metal compound. Useful metal compounds containing oxygen atoms and having at least two oxidation states of the metal of approximately equal energies include transition metal oxides such as titanium dioxide, vanadium oxide, chromium oxide, manganese oxide, iron oxide, nickel oxide, cobalt oxide, and rare earth metal oxides such as cerium oxide, praseodymium oxide, etc. Oxides of the metals are preferable because the ceramic properties thereof provide relatively long useful lives at higher operating temperatures and of the inherent presence of oxygen atoms. Other compounds and mixtures of the oxides with each other and with the other compounds also can be used. Energies of the two oxidation states of the metals must be sufficiently close to permit reversal by changes in the equilibrium oxygen pressure of the gases at operating temperature. Simple empirical tests may be used to determine the required relationship. The electrodes are attached to a surface of the plate or embedded with the plate. One preferred construction involves sandwiching the electrodes between two green ceramic plates and firing the assembly into a unitary structure.
Maintaining the sensing member within a relatively broad temperature range, typically about 600.degree.-900.degree.C., produces adequate indications of the air-fuel ratio supplied to an engine despite the fact that temperature variations change the resistance between the electrodes. Temperatures below 600.degree.C. tend to coat the member with soot and other particulate impurities while temperatures above 900.degree.C. tend to decrease overall life. Accuracy improvements are achieved by associating a controlled electrical heater with the sensing member to maintain its temperature within a narrower range. A highly useful structure involves a sandwich made of three green ceramic plates with the electrodes between an outer plate and the middle plate and an electrical resistance wire between the middle plate and the other outer plate. A thermocouple for temperature control can be embedded with either the electrodes or the resistance wire.
It is believed that the metal ions of the metal compounds are reduced or oxidized from one oxidation state to the other in proportion to the reducing or oxidizing nature of the exhaust gases. In the case of titanium dioxide molecules, for example, reduction frees an electron that conducts current much more readily and thereby reduces the resistance of the portion of the ceramic material located between the electrical leads.
In order for the sensing member to operate effectively in the automotive environment the sensing member must be capable of withstanding the temperature extremes and the thermal cycling normally encountered in the exhaust system and must demonstrate a response time which is at least as rapid as the response time demonstrated by the slowest engine component or by the transport properties of the fluid medium and the engine. For example, the sensor must demonstrate a response time no slower than 1 second, and preferably on the order of about 0.1 second or faster, in recognizing and responding to a change in the exhaust gas chemistry.
Ceramic materials are generally recognized as being compatible with the temperature extremes and the temperature excursions normally encountered in the engine exhaust system. In order to demonstrate the requisite service life requirement, however, the sensor response must not vary noticeably over its service life. This requires that the ceramic be chemically stable and not demonstrate substantial grain growth during its service life since this would alter its electrical properties. Furthermore, the ceramic material must demonstrate substantial strength since the automotive exhaust system environment is a mechanically harsh environment wherein substantial stressing through vibration and thermal shock and cycling may be encountered. It is therefore an object of the present invention to provide a ceramic sensor suitable for use in the exhaust system of an automotive vehicle internal combustion engine. In order to provide a suitable sensor, the ceramic used must be sufficiently porous that the exhaust gases will readily permeate the sensor, while the sensor must be sufficiently strong to withstand the exhaust environment. It is therefore a specific object of this invention to provide a partial pressure of oxygen sensor having high porosity and improved strength. More particularly, it is an object of this invention to provide a ceramic, resistive type partial pressure of oxygen sensor having high porosity and improved strength. Our above-noted co-pending applications describe a method of forming a ceramic exhaust gas sensor in the form of a substantially uniform and solid pellet of ceramic material which is capable of withstanding the automotive enviroment. The sensor theredescribed demonstrates a strong switching characteristic, that is, the resistance of the ceramic body varies over a wide range, as the air-fuel ratio of the combustion mixture varies slightly from the stoichiometric ratio. Such a sensor is of great utility when it is desired to operate the associated engine at the stoichiometric mixture ratio. However, nonstoichiometric operation of the associated engine causes the sensor to operate in a region of resistance where the changes of resistance in response to mixture ratio changes is slight and approximately linear. Sensors fabricated in accord with the broad teachings of the above-noted applications do not provide sufficiently repeatable results. It is therefore a specific objective of the present invention to provide an improved porous transition metal oxide ceramic sensor capable of reliable operation at nonstoichiometric mixture ratios. It is also an object of the present invention to provide a ceramic exhaust gas sensor which provides repeatable results, from sensor to sensor and during the life of any one sensor, when the air-fuel ratio is desired to be nonstoichiometric. It is a still further object of the present invention to provide such a sensor which has a response time of less than one second.
The exhaust gas sensor described in the above-noted patent applications has many desirable attributes. For example, with the exception of the various electrical wires and electrodes it is a porous, relatively small ceramic mass capable of being formed in various sizes and configurations. It does not require the use of any dissimilar materials such as housing, substrate, or interspersed particles. It is also fully immersed in the gas being sensed and does not require exposure to a reference. It is therefore a further object of the present invention to provide a sensor having the hereinbefore mentioned objects and having the further object of being comprised of an essentially uniform and porous ceramic body. It is also an object of the present invention to provide a method of producing such a sensor.